Avoid divide-by-zero crash in image filtering

[lukegb]

Changes proposed in this pull request:

• Avoid divide-by-zero crash in image filtering - although a kernel size of 1 is pointless/trivial, but it shouldn't cause the C extension to crash.

Fixes a crash accidentally introduced by the validation logic added in dff01a0.

[codspeed-hq]

Merging this PR will not alter performance

✅ 345 untouched benchmarks
⏩ 2 skipped benchmarks¹

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_{Comparing lukegb:pls-no-fpe (9f7010d) with main (1cf685c)}

Footnotes

1. 2 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[radarhere]

I've created lukegb#3 with a suggestion - rather than expand the image by zero, I think just copying the image would be more efficient.

You probably want to keep margin > 0 for defensiveness.